1. Field of the Invention
The present invention relates to impedance conversion circuits and antenna devices including impedance conversion circuits.
2. Description of the Related Art
In recent years, it is desired that communication terminal devices such as cellular phones are compatible with a wide variety of communication systems. In such communication terminal devices, antenna devices need to cover a wide range of frequency bands, such as, for example, from 800 MHz to 2.4 GHz.
For example, Japanese Patent No. 4935956 discloses such an antenna device compatible with a wide frequency band. FIG. 6 is one example of a circuit diagram of the antenna device disclosed in the Japanese Patent No. 4935956. An impedance conversion circuit 25 of this antenna device includes a primary coil composed of a coil element L1a and a coil element L1b and a secondary coil composed of a coil element L2a and a coil element L2b. A first end portion of the coil element L1a is connected to a feeding circuit 30, a second end portion of the coil element L1b is connected to an antenna element 11, a first end portion of the coil element L2b is connected to the antenna element 11, and a second end portion of the coil element L2a is connected to ground. FIG. 7 is a diagram illustrating an example of conductor patterns at respective layers in a case where the impedance conversion circuit illustrated in FIG. 6 is formed as a multilayer substrate. Conductor patterns that form the coil element L1a, the coil element L1b, the coil element L2a, and the coil element L2b are formed at a plurality of dielectric layers, and those conductor patterns are connected across the layers with a number of via conductors. Further, the coil element L1a is connected in series to the coil element L1b, and similarly the coil element L2a is connected in series to the coil element L2b. 
In the antenna devices including impedance conversion circuits, downsizing of the impedance conversion circuit itself is desirable since electronic devices such as cellular phones and the like, in which the antenna devices are incorporated, are being downsized, and the available space for the antenna device is limited. However, as illustrated in FIG. 7, the impedance conversion circuit, in which the conductor patterns are formed in such a way that the coil elements are sequentially connected in series across a plurality of layers, has a larger thickness in a stacking direction of the dielectric layers.
To downsize the impedance conversion circuit itself, it is preferable that the smallest possible number of turns is used in the coils, and the smallest possible number of layers is used in the dielectric layers. However, the impedance conversion ratio of the impedance conversion circuit is determined basically by the number of turns in the primary coil and the secondary coil. Thus, in the case where the number of turns is smaller, it may be difficult to obtain a desired transformer ratio. Further, as the number of turns is reduced, the percentages of inductance at input and output portions, which does not contribute to the coupling of transformer, become larger. This creates another issue in obtaining a coupling factor between the primary coil and the secondary coil.
To obtain a predetermined (larger) coupling factor with even smaller coils, it is desirable to use an identical shape (nearly congruent shape) for the primary coil and the secondary coil and arrange them to overlap on top of each other. In the case where the identical shape is used for the primary coil and the secondary coil, however, it may be very difficult to obtain desired inductance values for the primary coil and the secondary coil.